1. Field of the Invention
The invention pertains to the field of optoelectronic devices. More particularly, the invention pertains to edge-emitting lasers, surface emitting lasers, wavelength-tunable lasers, optical amplifiers, photodetectors, optical switches, and light-emitting diodes.
2. Description of Related Art
A prior art semiconductor diode laser, or more specifically, an edge-emitting laser, is shown in FIG. 1(a). The laser structure (100) is grown epitaxially on an n-doped substrate (101). The structure further includes an n-doped cladding layer (102), a waveguide (103), a p-doped cladding layer (108), and a p-contact layer (109). The waveguide (103) includes an n-doped part (104), a confinement layer (105) with an active region (106) inside the confinement layer, and a p-doped part (107). The n-contact (111) is contiguous with the substrate (101), and a p-contact (112) is mounted on the p-contact layer (109). The active region (106) generates light when a forward bias (113) is applied. The profile of the optical mode in the vertical direction (z) is determined by the refractive index profile in the z-direction.
The waveguide (103) is bounded in the lateral plane by a front facet (116) and a rear facet (117). If a special highly reflecting coating is put on the rear facet (117), the laser light (115) is emitted only through the front facet (116).
The substrate (101) is preferably formed from any III-V semiconductor material or Ill-V semiconductor alloy, e.g. GaAs, InP, or GaSb. GaAs or InP are generally used depending on the desired emitted wavelength of laser radiation. The substrate (101) is preferably either a slice of a bulk crystal or an epilayer deposited on the slice of a bulk crystal. Alternatively, sapphire, SiC or Si (111) can be used as substrates for GaN-based lasers, i.e. laser structures the layers of which are formed of GaN, AlN, InN, or alloys of these materials. The substrate (101) is doped by an n-type, or donor impurity. Possible donor impurities include, but are not limited to S, Se, Te, and amphoteric impurities like Si, Ge, or Sn where the latter are introduced under such technological conditions that they are incorporated predominantly into the cation sublattice and serve as donor impurities.
The n-doped cladding layer (102) is preferably formed from a material lattice-matched or nearly lattice-matched to the substrate (101), is transparent to the generated light, and is doped by a donor impurity. In the case of a GaAs substrate (101), the n-doped cladding layer is preferably formed from a GaAlAs alloy or an In(Ga,Al)P alloy lattice matched or nearly lattice matched to a GaAs substrate.
The n-doped part (104) of the waveguide (103) is preferably formed from a material lattice-matched or nearly lattice-matched to the substrate (101), is transparent to the generated light and is doped by a donor impurity. In the case of a GaAs substrate, the n-doped part (104) of the waveguide is preferably formed of a GaAlAs alloy having an Al content lower than that in the n-doped cladding layer (102), or of GaAs.
The p-doped part (107) of the waveguide (103) is preferably formed from a material lattice-matched or nearly lattice-matched to the substrate (101), is transparent to the generated light and is doped by an acceptor impurity. Preferably, the p-doped part (107) of the waveguide is formed from the same material as the n-doped part (104), but doped by an acceptor impurity. Possible acceptor impurities include, but are not limited to, Be, Mg, Zn, Cd, Pb, Mn and amphoteric impurities like Si, Ge, or Sn where the latter are introduced under such technological conditions that they are incorporated predominantly into the anion sublattice and serve as acceptor impurities.
The p-doped cladding layer (108) is preferably formed from a material lattice-matched or nearly lattice-matched to the substrate (101), is transparent to the generated light, and is doped by an acceptor impurity. Preferably, the p-doped cladding layer (108) is formed from the same material as the n-doped cladding layer (102), but is doped by an acceptor impurity.
The p-contact layer (109) is preferably formed from a material lattice-matched or nearly lattice matched to the substrate, is transparent to the generated light and is doped by an acceptor impurity. The doping level is preferably higher than that in the p-cladding layer (108).
The metal contacts (111) and (112) are preferably formed from multi-layered metal structures. The metal contact (111) is preferably formed from the structures including, but not limited to, the structure Ni—Au—Ge. Metal contacts (112) are preferably formed from structures including, but not limited to, the structure Ti—Pt—Au.
The confinement layer (105) is preferably formed from a material lattice-matched or nearly lattice-matched to the substrate (101), is transparent to the generated light, and is undoped or weakly doped. The confinement layers are preferably formed from the same material as the substrate (101).
The active region (106) placed within the confinement layer (105) is preferably formed by any insertion, the energy band gap of which is narrower than that of the substrate (101). Possible active regions (106) include, but are not limited to, a single-layer or a multi-layer system of quantum wells, quantum wires, quantum dots, or any combination thereof. In a device on a GaAs-substrate, examples of the active region (106) include, but are not limited to, a system of insertions of InAs, In1-xGaxAs, InxGa1-x-yAlyAs, InxGa1-xAs1-yNy or similar materials.
One of the major shortcomings of the edge-emitting laser is a variation of the energy band gap with temperature resulting in an undesirable temperature dependence of the wavelength of emitted light, particularly for high output power operation.
FIG. 1(b) shows schematically a prior art surface emitting laser, particularly, a vertical cavity surface emitting laser (VCSEL) (120). The active region (126) is put into a cavity (123), which is sandwiched between an n-doped bottom mirror (122) and a p-doped top mirror (128). The cavity (123) includes an n-doped layer (124), a confinement layer (125), and a p-doped layer (127). Bragg reflectors each including a periodic sequence of alternating layers having low and high refractive indices are used as a bottom mirror (122) and a top mirror (128). The active region (126) generates light when a forward bias (113) is applied. Light comes out (135) through the optical aperture (132). The wavelength of the emitted laser light from the VCSEL is determined by the length of the cavity (123).
The layers forming the bottom mirror (122) are preferably formed from materials lattice-matched or nearly lattice matched to the substrate (101), are transparent to the generated light, are doped by a donor impurity and have alternating high and low refractive indices. For a VCSEL grown on a GaAs substrate, alternating layers of GaAs and GaAlAs or layers of GaAlAs having alternating aluminum content preferably form the mirror.
The n-doped part (124) of the cavity (123) is preferably formed from a material lattice-matched or nearly lattice-matched to the substrate (101), is transparent to the generated light and is doped by a donor impurity.
The p-doped part (127) of the cavity (123) is preferably formed from a material lattice-matched or nearly lattice-matched to the substrate (101), is transparent to the generated light and is doped by an acceptor impurity.
The layers forming the top mirror (128) are preferably formed from materials lattice-matched or nearly lattice-matched to the substrate (101), are transparent to the generated light, are doped by an acceptor impurity and have alternating high and low refractive indices. For a VCSEL grown on a GaAs substrate, alternating layers of GaAs and GaAlAs or layers of GaAlAs having alternating aluminum content preferably form the mirror.
The p-contact layer (129) is preferably formed from a material doped by an acceptor impurity. For a VCSEL grown on a GaAs substrate, the preferred material is GaAs. The doping level is preferably higher than that in the top mirror (128). The p-contact layer (129) and the metal p-contact (112) are etched to form an optical aperture (132). The doping profile is optimized to reach a possibly low resistance of the device.
The confinement layer (125) is preferably formed from a material lattice-matched or nearly lattice-matched to the substrate (101), is transparent to the generated light, and is undoped or weakly doped. The confinement layers are preferably formed from the same material as the substrate (101).
The active region (126) placed within the confinement layer (125) is preferably formed by any insertion, the energy band gap of which is narrower than that of the substrate (101). Possible active regions (126) include, but are not limited to, a single-layer or a multi-layer system of quantum wells, quantum wires, quantum dots, or any combination thereof. In a device on a GaAs-substrate, examples of the active region (126) include, but are not limited to, a system of insertions of InAs, In1-xGaxAs, InxGa1-x-yAlyAs, InxGa1-xAs1-yNy or similar materials.
The active region (126) generates optical gain when a forward bias (113) is applied. The active region (126) then emits light which is bounced between the bottom mirror (122) and the top mirror (128). The mirrors have high reflectivity for light propagating in the direction normal to the p-n junction plane, and the reflectivity of the bottom mirror (122) is preferably higher than that of the top mirror (128). Thus, the VCSEL design provides a positive feedback for light propagating in the vertical direction and finally results in lasing. The laser light (135) comes out through the optical aperture (132).
One of the major advantages of a VCSEL is temperature stabilization of the wavelength. Temperature variations of the wavelength follow the temperature variations of the refractive index, which is an order of magnitude smaller than the variations of the semiconductor band gap energy. One severe disadvantage of a VCSEL is that its output power is limited by a few mW, as it is not possible to provide efficient heat dissipation in the VCSEL geometry.